The present invention is concerning an apparatus for processing of high viscosity materials, comprising a heatable, horizontal vessel with baffle elements and scraping elements, and, supported on a rotating hollow shaft, a plurality of heatable stirrer elements. In particular, the invention is referring to an apparatus for the production, from preformed condensates, of polycondensates, through melt-phase polycondensation, at elevated temperature and under vacuum. The apparatus also can be used in the chemical treatment of polymer melts, for example, the conversion of end groups with modifying agents.